It is known to pass a gas flow through an air permeable wall to remove dirt from the gas flow. The gas permeable wall, such as a filter, can include holes or perforations of a size smaller than the size of the dirt particles. Consequently, the dirt particles are not capable of passing through the gas permeable wall together with the gas flow.
An issue involved with the above mentioned method is the need for maintenance. The dirt which is incapable of passing through the gas permeable wall can remain stuck to the wall and as the amount of dirt increases it can become difficult for the gas flow to pass through the layer of accumulated dirt.
In order to avoid the gas flow through the gas permeable wall decreasing; the accumulated dirt should be removed. This can be done by changing or cleaning the filter. In other words the gas permeable wall can involve maintenance personal to carry out the necessary tasks. In addition, the apparatus may need to be taken out of use for the time period necessary to carry out the maintenance tasks.